Metformin has long been feared for its potential to cause fatal lactic acidosis when administered to patients with compromised renal function. Hospitalized patients with depreciated renal function, particularly patients with cancer and diabetes, are usually switched to insulin upon admission and some of them continue receiving insulin beyond that hospital visit. However, insulin is a growth hormone known to promote differentiation, transformation, multiplication and survival, as well as to prevent apoptosis of many tumor cell lines. In contrast, studies have demonstrated that metformin reduces tumor cell proliferation and induces their apotosis. Thus, reinitiating metformin instead of switching to insulin in patients with diabetes and cancer may in fact increase their survival. We aimed to assess whether different doses and frequencies of metformin could be safely reinitiated in patients with various types of cancer and diabetes with mild, moderate, and severe renal function impairment